The countermanding task probes monkeys' ability to control behavior by requiring them to withhold planned movements. To succeed in this task monkeys must monitor and control their performance. This laboratory has used the countermanding task to study the function of three frontal areas. Cells in frontal eye field (FEF) generate signals sufficient to control the production of saccades (Hanes et al., J Neurophysiol 79:817, 1998). In contrast, neurons in supplementary eye field (SEF) register the occurrence of errors, conflict, and reward (Stuphorn et al., Nature 408:857,2000). We also recorded from anterior cingulate cortex (ACC) of two macaque monkeys performing the countermanding task, since the ACC is believed to be the source of behavioral monitoring in humans. Indeed we found signals that monitor performance similar to the neurons in SEF (Stuphorn and Schall, SfN Abstract 26:404.5, 2000). Here we report that one particular group of these neurons showed an increase of activity after non-canceled saccades...